To decrease the weight of a small internal combustion engine mounted in a two-wheeled motor vehicle, a three-wheeled motor vehicle, an all terrain vehicle, a personal watercraft, etc., the recent trend in the manufacture of small internal combustion engines is toward thinning a separating wall of a crankcase to an extent that stiffness of the crankcase is not substantially reduced.
When the internal combustion engine stops from a running state, oil may move downward in an oil passage, and air may flow into the oil passage from above. Therefore, when the engine re-starts, the air may be mixed into a lubricating oil to be fed. To increase lubrication ability or efficiency, it is desirable to avoid entry of the air into the lubricating oil. Such prior art is disclosed in Japanese Utility Model Examined Application Publication No. Sho. 61-18166.